I certainly wouldn't go any smaller than a 75. The issue you run into with smaller tanks is room to turn. Comets can easily reach 10 inches, and a 10-inch fish trying to turn around in a 12 inch wide tank is going to be an issue. A 75 gal adds another 5 inches of width. A 75 gal is eventually the size I'd like to end up with for my fancy goldfish. A little extra turn room helps everybody.

As far as the GPH for canisters I can't speak to that. I just know that canisters have a larger capacity for media, are quieter, and are simply better-built filters. The construction of them makes it so the water hits the filter media first and then the impeller; this makes it harder for sand to ruin it. I'm pretty sure I've burned out at least one HOB impeller with sand. And goldfish love to throw that stuff around more than loaches like to root around in it.

Some stores will sell comets at about 5-6 inches. A garden store would be your best bet to find them at that size and for a more reasonable price than a petstore.

EDIT:
I fed my goldfish tiny pieces of carrot, peas, blanched lettuce (romaine), cucumber, zucchini, small pieces of shrimp.. And I think that's about it. Just be careful, just because something fits in their mouth, doesn't mean that they can actually swallow it. I had to save a comet from himself. lol

And rinse, rinse, rinse your sand. if there's ANY silt left, it'll get into your filter as the fish stirs it up, and it will mess with your mechanical pieces to your filters.

You know what guys...I have changed my mind. I'm going to get an oscar cichlid instead...A 75 gallon is suitable for them, they get bigger than goldfish in general (at least their bodies) and I can keep just one with no guilt...Some experienced people have told me I can keep one goldfish alone, but some other experienced people have told me otherwise...So I really can't keep just one goldfish without feeling that lingering guilt....

I think an Oscar would be more suited for me. Of course that adds a heater to the plans...lol